chore(core): Deprecate km_core_keyboard_load

[ermshiperete]

#12459 added the new core API method km_core_keyboard_load_from_blob. Once we changed the engines to use the new method we should add the deprecated flag to km_core_keyboard_load.

• refactor(linux): implement loading of keyboard blob in Keyman for Linux #12498
• refactor(mac): implement loading of keyboard blob in Keyman for Mac #12499
• refactor(windows): implement loading of keyboard blob in Keyman for Windows #12500
• refactor(developer): implement loading of keyboard blob in Keyman Developer #12690
• add deprecated flag to km_core_keyboard_load
• #ifdef any filesystem code so it does not compile to WASM

[mcdurdin]

Added WASM TODO item

[ermshiperete]

Needs to happen after the Linux/Mac/Windows subtasks are done.

[mcdurdin]

Note: once mac work is done (#12499), we can remove the filesystem code entirely from Core, rather than #ifdef or deprecation of the API. This will be much cleaner!

[mcdurdin]

As we are so close to getting this finished, brought it back to milestone 18.0.